In the drive toward higher density recording media, spin filters have become of interest for use in magnetoresistive (MR) read heads. FIG. 1 is a diagram of a conventional spin filter 10 that may be used in reading high density recording media. In general, the conventional spin filter 10 would be incorporated into a MR read head (not explicitly shown), which would include leads and other electronics to drive current through the conventional spin filter 10 during reading. In such an application, current is generally driven in the current perpendicular to the plane (CPP) configuration. The CPP configuration is in the z-direction depicted in FIG. 1.
The conventional spin filter 10 includes a conventional seed layer 20, a conventional antiferromagnetic (AFM) layer 30, a conventional pinned layer 40, a conventional nonmagnetic spacer layer 50, a conventional free layer 60, a conventional diffusion barrier 70, a Cu layer 80, and a conventional capping layer 90. The conventional seed layer 20 is used to provide the appropriate surface for growing the conventional AFM layer 30 with the desired crystal structure. The conventional AFM layer 30 is used in pinning the magnetization of the conventional pinned layer 40. The conventional pinned layer 40 may be a synthetic pinned layer, including conventional ferromagnetic layers 42 and 46 separated by a conductive spacer layer 44 that is typically Ru. The conductive spacer layer 44 has a thickness configured to ensure that the ferromagnetic layers 42 and 46 are antiferromagnetically coupled. The conventional nonmagnetic spacer layer 50 is typically conductive, for example Cu. The conventional free layer 60 is ferromagnetic. The conventional free layer 60 may be a bilayer, including a lower CoFe layer and a NiFe layer on the CoFe layer. The Cu layer 80 is a conventional spin filter layer. The Cu layer 80 is a conventional spin filter layer because the Cu layer 80 improves the scattering of electrons from the free layer 60 that are incident on the capping layer 90, discussed above. The capping layer 90 is typically Ta. After formation, an upper portion of the Ta capping layer 90 oxidizes, providing a specular reflection layer (not explicitly shown) within the upper portion of the capping layer 90. Also included is a Ru diffusion barrier 70 between the free layer 60 and the Cu layer 80.
Although the conventional spin filter 10 functions, one of ordinary skill in the art will readily recognize that there are drawbacks to the use of the conventional spin filter 10. Oxygen may diffuse through the Ta layer 90 and the Cu layer 80. In the absence of the Ru diffusion barrier 70, this oxygen may reach the free layer 60 and degrade the thermal stability of the conventional spin filter 10. Although the Ru diffusion barrier 70 prevents oxygen from reaching the free layer 60, one of ordinary skill in the art will readily recognize that there are issues with the use of the Ru diffusion barrier 70. In particular, the resistance of the spin filter is lowered because of the relationship between the materials Ru and Cu. Other conventional approaches substitute materials such as Ag and Au for the Cu layer 80 and omit the Ru diffusion barrier 70. However, one of ordinary skill in the art will readily recognize that such conventional spin filters are subject to interdiffusion with the NiFe layer at the top of the free layer 60. Consequently, the signal from such a may be reduced.
U.S. Pat. No. 6,586,121 (Ide) discloses another conventional spin filter. In the conventional spin filter of Ide, the Cu layer 80 and the capping layer 90 are replaced by layers composed of at least one of Ru, Pt, Ir, Rh, Pd, Os, and Cr. Although such a conventional spin filter may address some of the issues described above, the signal from such a spin filter may be degraded.
Accordingly, what is needed is a system and method for providing a spin filter having improved thermal stability while preserving the signal for the conventional spin filter. The present invention addresses such a need.